Fixes as per review.

[rust-101.git] / src / part12.rs

// Rust-101, Part 12: Rc, Interior Mutability, Cell, RefCell
// =========================================================

use std::rc::Rc;
use std::cell::{Cell, RefCell};

//@ Our generic callback mechanism is already working quite nicely. However, there's one point we
//@ may want to fix: `Callbacks` does not implement `Clone`. The problem is that closures (or
//@ rather, their environment) can never be cloned.
//@ (There's not even an automatic derivation happening for the cases where it would be possible.)
//@ This restriction propagates up to `Callbacks` itself. What could we do about this?

//@ ## `Rc`
//@ The solution is to find some way of cloning `Callbacks` without cloning the environments. This
//@ can be achieved with `Rc<T>`, a *reference-counted* pointer. This is is another example of a
//@ smart pointer. You can `clone` an `Rc` as often as you want, that doesn't affect the data it
//@ contains. It only creates more references to the same data. Once all the references are gone,
//@ the data is deleted.
//@ 
//@ Wait a moment, you may say here. Multiple references to the same data? That's aliasing! Indeed:
//@ Once data is stored in an `Rc`, it is read-only and you can only ever get a shared reference to the data again.

//@ Because of this read-only restriction, we cannot use `FnMut` here: We'd be unable to call the
//@ function with a mutable reference to it's environment! So we have to go with `Fn`. We wrap that
//@ in an `Rc`, and then Rust happily derives `Clone` for us.
#[derive(Clone)]
struct Callbacks {
    callbacks: Vec<Rc<Fn(i32)>>,
}

impl Callbacks {
    pub fn new() -> Self {
        Callbacks { callbacks: Vec::new() }
    }

    // Registration works just like last time, except that we are creating an `Rc` now.
    pub fn register<F: Fn(i32)+'static>(&mut self, callback: F) {
        self.callbacks.push(Rc::new(callback));                     /*@*/
    }

    pub fn call(&self, val: i32) {
        // We only need a shared iterator here. Since `Rc` is a smart pointer, we can directly call the callback.
        for callback in self.callbacks.iter() {
            callback(val);                                          /*@*/
        }
    }
}

// Time for a demo!
fn demo(c: &mut Callbacks) {
    c.register(|val| println!("Callback 1: {}", val));
    c.call(0); c.clone().call(1);
}

pub fn main() {
    let mut c = Callbacks::new();
    demo(&mut c);
}

// ## Interior Mutability
//@ Of course, the counting example from last time does not work anymore: It needs to mutate the
//@ environment, which a `Fn` cannot do. The strict borrowing Rules of Rust are getting into our
//@ way. However, when it comes to mutating a mere number (`usize`), there's not really any chance
//@ of problems coming up. Everybody can read and write that variable just as they want.
//@ So it would be rather sad if we were not able to write this program. Lucky enough, Rust's
//@ standard library provides a solution in the form of `Cell<T>`. This type represents a memory
//@ cell of some type `T`, providing the two basic operations `get` and `set`. `get` returns a
//@ *copy* of the content of the cell, so all this works only if `T` is `Copy`.
//@ `set`, which overrides the content, only needs a *shared reference* to the cell. The phenomenon
//@ of a type that permits mutation through shared references (i.e., mutation despite the
//@ possibility of aliasing) is called *interior mutability*. You can think of `set` changing only
//@ the *contents* of the cell, not its *identity*. In contrast, the kind of mutation we saw so far
//@ was about replacing one piece of data by something else of the same type. This is called
//@ *inherited mutability*. <br/>
//@ Notice that it is impossible to *borrow* the contents of the cell, and that is actually the key
//@ to why this is safe.

// So, let us put our counter in a `Cell`, and replicate the example from the previous part.
fn demo_cell(c: &mut Callbacks) {
    {
        let count = Cell::new(0);
        // Again, we have to move ownership of the `count` into the environment closure.
        c.register(move |val| {
            // In here, all we have is a shared reference of our environment. But that's good enough
            // for the `get` and `set` of the cell!
            //@ At run-time, the `Cell` will be almost entirely compiled away, so this becomes
            //@ pretty much equivalent to the version we wrote in the previous part.
            let new_count = count.get()+1;
            count.set(new_count);
            println!("Callback 2: {} ({}. time)", val, new_count);
        } );
    }

    c.call(2); c.clone().call(3);
}

//@ It is worth mentioning that `Rc` itself also has to make use of interior mutability: When you
//@ `clone` an `Rc`, all it has available is a shared reference. However, it has to increment the
//@ reference count! Internally, `Rc` uses `Cell` for the count, such that it can be updated during
//@ `clone`.
//@ 
//@ Putting it all together, the story around mutation and ownership through references looks as
//@ follows: There are *unique* references, which - because of their exclusivity - are always safe
//@ to mutate through. And there are *shared* references, where the compiler cannot generally
//@ promise that mutation is safe. However, if extra circumstances guarantee that mutation *is*
//@ safe, then it can happen even through a shared reference - as we saw with `Cell`.

// ## `RefCell`
//@ As the next step in the evolution of `Callbacks`, we could try to solve this problem of
//@ mutability once and for all, by adding `Cell` to `Callbacks` such that clients don't have to
//@ worry about this. However, that won't end up working: Remember that `Cell` only works with
//@ types that are `Copy`, which the environment of a closure will never be. We need a variant of
//@ `Cell` that allows borrowing its contents, such that we can provide a `FnMut` with its
//@ environment. But if `Cell` would allow that, we could write down all those crashing C++
//@ programs that we wanted to get rid of.
//@ 
//@ This is the point where our program got too complex for Rust to guarantee at compile-time that
//@ nothing bad will happen. Since we don't want to give up the safety guarantee, we are going to
//@ need some code that actually checks at run-time that the borrowing rules are not violated. Such
//@ a check is provided by `RefCell<T>`: Unlike `Cell<T>`, this lets us borrow the contents, and it
//@ works for non-`Copy` `T`. But, as we will see, it incurs some run-time overhead.

// Our final version of `Callbacks` puts the closure environment into a `RefCell`.
#[derive(Clone)]
struct CallbacksMut {
    callbacks: Vec<Rc<RefCell<FnMut(i32)>>>,
}

impl CallbacksMut {
    pub fn new() -> Self {
        CallbacksMut { callbacks: Vec::new() }
    }

    pub fn register<F: FnMut(i32)+'static>(&mut self, callback: F) {
        let cell = Rc::new(RefCell::new(callback));                 /*@*/
        self.callbacks.push(cell);                                  /*@*/
    }

    pub fn call(&mut self, val: i32) {
        for callback in self.callbacks.iter() {
            // We have to *explicitly* borrow the contents of a `RefCell` by calling `borrow` or
            // `borrow_mut`.
            //@ At run-time, the cell will keep track of the number of outstanding shared and
            //@ mutable references, and panic if the rules are violated. <br />
            //@ For this check to be performed, `closure` is a *guard*: Rather than a normal
            //@ reference, `borrow_mut` returns a smart pointer ([`RefMut`](https://doc.rust-
            //@ lang.org/stable/std/cell/struct.RefMut.html), in this case) that waits until is
            //@ goes out of scope, and then appropriately updates the number of active references.
            //@ 
            //@ Since `call` is the only place that borrows the environments of the closures, we
            //@ should expect that the check will always succeed, as is actually entirely useless.
            //@ However, this is not actually true. Several different `CallbacksMut` could share a
            //@ callback (as they were created with `clone`), and calling one callback here could
            //@ trigger calling all callbacks of the other `CallbacksMut`, which would end up
            //@ calling the initial callback again.
            //@ This issue of functions accidentally recursively calling themselves is called
            //@ *reentrancy*, and it can lead to subtle bugs. Here, it would mean that the closure
            //@ runs twice, each time thinking it has a unique, mutable reference to its
            //@ environment - so it may end up dereferencing a dangling pointer. Ouch!
            //@ Lucky enough, Rust detects this at run-time and panics once we try to borrow the
            //@ same environment again. I hope this also makes it clear that there's absolutely no
            //@ hope of Rust performing these checks statically, at compile-time: It would have to
            //@ detect reentrancy!
            let mut closure = callback.borrow_mut();
            // Unfortunately, Rust's auto-dereference of pointers is not clever enough here. We
            // thus have to explicitly dereference the smart pointer and obtain a mutable reference
            // to the content.
            (&mut *closure)(val);
        }
    }
}

// Now we can repeat the demo from the previous part - but this time, our `CallbacksMut` type
// can be cloned.
fn demo_mut(c: &mut CallbacksMut) {
    c.register(|val| println!("Callback 1: {}", val));
    c.call(0);

    {
        let mut count: usize = 0;
        c.register(move |val| {
            count = count+1;
            println!("Callback 2: {} ({}. time)", val, count);
        } );
    }
    c.call(1); c.clone().call(2);
}

// **Exercise 12.1**: Write some piece of code using only the available, public interface of
// `CallbacksMut` such that a reentrant call to a closure is happening, and the program panics
// because the `RefCell` refuses to hand out a second mutable borrow of the closure's environment.

//@ [index](main.html) | [previous](part11.html) | [raw source](workspace/src/part12.rs) |
//@ [next](part13.html)